1. Field of the Invention
The present invention relates to the after-treatment of gases emitted in the exhaust of Diesel vehicles.
2. Description of the Prior Art
Particulate emission standards have recently come into effect in Europe. These standards will become more stringent in the coming years. By that time, improvements linked with engines and fuels may be insufficient, even in the presence of an oxidation catalytic muffler, especially for the soluble part of the particulate emitted.
Particulate filters are a well-known exhaust gas after-treatment technique. It is thus possible to obtain filtration efficiencies above 80%. Many filter technologies have been developed to date. Examples thereof are the ceramic monolith marketed by the Coming Company, or the cartridge with coiled ceramic fibers as described in patent application WO-95/127,843.
The technical difficulty encountered for developping such a system lies in that the filter must be periodically regenerated by combustion of the soot deposit. This combustion sometimes occurs naturally when the temperature of the gases reaches by itself the level required to initiate oxidation of the particulate matter. However, average running conditions generally lead to temperatures that are too low to spontaneously initiate combustion of the particulates. This leads to clogging of the filter, which is inconvenient for the engine efficiency and eventually threatens the running thereof It is then necessary to provide artificial regeneration of the filter.
Many techniques have been developped to that effect. They can be based on changes in the running of the engine:intake throttling, exhaust throttling, advanced injection lag, or linked with energy supply in the exhaust gases or at the level of the filter (resistor, burner, micro-wave, . . . ). It is then necessary to control these various devices by means of an outer control driven by a computer. Most often, the criterion taken into account for regeneration initiation is the back pressure in the exhaust line.
In order to facilitate regeneration of particulate filters, a different approach of chemical nature consists in adding to the fuel an additive, for example an organometallic additive that is found in the soot deposit, which generally leads to a decrease in the ignition temperature and therefore to a regeneration frequency increase.
Examples of the products most commonly used as additives are copper, iron, cerium, sodium, . . . Studies show that, in the presence of such additives, partial regenerations can occur spontaneously for relatively low exhaust gas temperatures (.about.200.degree. C.).
However, for certain driving modes, problems linked with the back pressure can subsist, so that an external energy supply such as electric heating can be necessary.
Concerning power consumption, most of the well-known systems using electric heating perform a global heating of the filtering element. This leads to a controlled high power consumption. Generally, the electrical power required to initiate total regeneration of the filter is high and often not compatible with the electrical resources on board the vehicle. European Patent EP-B1-0,485,179 illustrates a system based on this principle.
Furthermore, the regeneration conditions can highly depend on the fouling condition of the filter. The well-known electric heaters do not allow action according to the fouling of the filter. The present invention advantageously adapts the electrical power and the power supply in the filter to all the operating conditions of the vehicle. It also overcomes the problems of the prior art mentioned above.